Primary documents or source material are anything created or recorded near to the time and place of an event. They are not necessarily authoritative, "true" or reliable. They are first hand accounts, carrying all the biases and unintentional honesty of the recorder or institution that produces them.
A diary account of a day on which an event, perhaps mentioned in an offhand way, occurred. A photograph or illustration. A contemporary newspaper article(though the editorial process behind the newspaper publication moves such a document closer to "secondary" status). A popular song, or poster or advertisement. An official record, such as a census, birth certificate, marriage record or court document, etc.